The thermal stability of two strong acid sulphonated crosslinked polystyrene cation resins, Zeo-Karb 225 and Dowex 50, were studied as aqueous slurries at temperatures up to 200°C. and heating periods of up to 30 days in a static system. The loss in strong acid capacity, gain in phenolic capacity, decomposition products, and swelling weight were determined. Both cation resins were stable at temperatures up to 120°C. in the hydrogen form. At higher temperatures capacity loss was significant; Zeo-Karb 225 when heated in the hydrogen form at 200°C. lost all capacity within 3 days of heating. Investigations of the manner and rate of decomposition up to 200°C. are presented. The effect of degree of crosslinking, particle size, and nature of the sorbed counter ion on thermal decomposition was determined. Thermal stability increased as the degree of crosslinking decreased, was unaffected by the particle size, and increased as the hydrated radius of the sorbed counter ion decreased. No physical damage occurred when heating cation resins in water up to 200°C. for 30 days. The performance of cation resins at 120°C. and 150°C. in a recirculating water loop was investigated and the results compared with static experiments described above. Zeo-Karb 225, hydrogen form, appeared to be stable at 150°C. in recirculating experiments. No physical damage occurred at temperatures up to 15000. and flow rates of up to 22.92 gal.min.-1 ft.-2 . The Carman correlation for pressure drop in a packed bed of particles was found to account for the hydrodynamic behaviour of cation resin particles in a recirculating circuit. Suggestions for operating cation resins for high temperature condensate polishing in nuclear and conventional steam raising plants are included.